Wireless communications between electronic or electric apparatuses, such as cell phones, wireless LAN, short-distance, wireless Bluetooth™, etc., have remarkably progressed in recent years. As wireless communications apparatuses have increasingly been integrated and become multi-functional, multi-band products having pluralities of communications systems, and products capable of transmitting large volume of signals, etc. have been developed. Among them, much attention is paid to a multiple-input, multiple-output (MIMO) wireless communications system. The MIMO communications system uses pluralities of transmitting antennas and pluralities of receiving antennas for the spatial-multiplexing transmission of plural signals in a predetermined frequency band, thereby increasing the volume of signals transmitted without expanding the used frequency band width. This system improves the utilization efficiency of frequency bands.
For instance, JP 2005-318115 A discloses a wireless communications system comprising (a) a receiver comprising pluralities of antennas, a means for branching signals received by part of the antennas, a means for multiplying each branched signal by a complex weight, a means for adding the received signals to output signals of the weight-multiplying means, a means for controlling the complex weight based on the output signals of the adding means such that a channel has the maximum capacity to the received signals, and RF front-ends for generating baseband signals based on the output signals of the adding means, and (b) a transmitter comprising pluralities of antennas, a means for generating transmitting signals, RF front-ends for generating RF transmitting signals based on the output signals of the transmitting-signal-generating means, a means for branching the RF transmitting signals, a means for multiplying the output of the branching means by a complex weight, a means for adding the outputs of the weight-controlling means, and a weight-controlling means for determining the complex weight based on a feedback signal from the receiver.
Because such MIMO communications system needs pluralities of antennas and the same number of RF front-ends as that of the antennas, it has a complicated structure with a large number of parts. Multi-band communications systems also suffer the same problems.
Thus desired are small, high-frequency circuit devices with a small number of parts, which can be used in multi-band communications systems, MIMO communications systems, etc.